Expression of many exogenous genes is readily obtained in a variety of recombinant host-vector systems, but becomes difficult to obtain if the protein normally requires extensive post-translational processing. This is the likely reason that expression in a fully recombinant system has not been reported for any of the major fibrillar collagens that require processing by eight or more post-translational enzymes. See Prockop and Kivirikko, N. Engl. J. Med., 311:376-386 (1984).
Schnieke et al., Proc. Natl. Acad Sci. U.S.A., 84:8869-8873 (1987) and Lee et al., J. Biol. Chem., 264:20683-20687 (1989), disclose rescue experiments in two different systems that synthesized only one of the two chains for type I procollagen. Schnieke et al. reported that a gene for the human fibrillar collagen pro.alpha.1(I) chain, the COL1A1 gene, can be expressed in mouse fibroblasts and the chains are used to assemble molecules of type I procollagen, the precursor of type I collagen. However, in this system the pro.alpha.2(I) chains found in the same molecule are of mouse origin. In the system of Lee et al. the pro.alpha.1(I) chains are of rat origin. Thus, synthesis of a procollagen molecule in which all three chains are derived from an exogenous gene was not obtained by either Schnieke et al. or Lee et al.
Failure to obtain expression of genes for fibrillar collagens in a fully recombinant system has hampered attempts to study the normal structure-function relationships of the proteins and to study the effects of mutations. In particular, mutations in the gene for type II procollagen have recently been implicated as the cause of several human diseases, Anderson et al., Am. J. Hum. Genet., 46:896-901 (1990); Tiller et al., Proc. Natl. Acad. Sci. U.S.A., 87:3889-3893, 1990; Vissing et al., J. Biol. Chem., 264:18265-18267 (1990); Lee et al., Science, 244:978-980 (1989); Francomano et al., Genomics, 1:293-296 (1987); Knowlton et al., Am. J. Hum. Genet., 45:681-688 (1989); Ahmad et al., Am. J. Hum. Genet., 47:A206 (1990); Palotie et al., The Lancet, I:924-927 (1989); Knowlton et al., N. Engl. J. Med., 322:526-530 (1990); Ala-Kokko, L., Baldwin et al., Proc. Natl. Acad. Sci. U.S.A., 87:6565-6568 (1990), but because adequate numbers of human cartilage cells are difficult to obtain and because human chondrocytes readily lose their phenotype in culture, Elima and Vuorio, FEBS Lett., 258:195-198 (1989); Aulthouse et al., In Vitro Dev. Biol., 25:659-668 (1989), the causal relationship between a mutation in the gene and the biological function of the protein has proven elusive.
Also, failure to obtain expression of genes for human fibrillar collagens has made it impossible to prepare human fibrillar procollagens and collagens that have a number of therapeutic uses in man and that will not produce the undesirable immune responses that have been encountered with use of collagen from animal sources.